An integrated circuit substantially comprises a semiconductor substrate incorporating electronic components and having a system of conductors interconnecting these components. The interconnect system may comprise a plurality of conductive layers, typically insulated from one another by dielectric layers, and connected to one another at certain points by vias that pass through the dielectric layers. Each conductive layer comprises numerous conductors parallel to one another in a direction orthogonal to the conductors of an adjacent conductive layer. In practice, the conductors are ordinarily strips, substantially made of aluminum, but other materials are beginning to be used, such as tungsten and silicides of titanium or tantalum.
The invention relates to integrated circuits in which metal strips of conductors of the interconnect system are provided with upper and lower films made of a material having a melting point substantially higher than that of the metal comprising the strips. Titanium-tungsten may be used for these films. An insulating passivation layer covers the upper conductive layer. Silicon nitride (Si.sub.3 N.sub.4) presently performs this function. The passivation layer generally serves to protect the integrated circuits beneath it mechanically, chemically and electrically.
Attempts are currently being made to make programmable connections of two superimposed conductors of the interconnect system using a laser. The connections, or links, are programmed for the reconfiguration of a circuit to overcome a defect, or for connecting a substitute circuit to a defective circuit using the technique of redundancy of the functional blocks on a chip.
The invention relates to methods that comprise applying a laser beam perpendicular to the layers of the interconnect system of the integrated circuit to make an opening in the insulating layer, an upper conductor, the dielectric layer and at least part of the lower conductor, and to form an electrical connection of the two conductors in this opening.
One known method of programmable connection by laser of two superimposed conductors is described in particular in the article by N. X. Platakis in the Journal of Applied Physics, Vol. 47, No. 5, May 1976, pages 2120-2128. At the point where the connection is desired, a laser beam is applied, the power, diameter, and number and duration of pulses of which are determined in such a way as to open up the upper conductor progressively, to eliminate the dielectric layer by explosion, and to form a contact between the two conductors by ejection of the melted material of the lower conductor up to the level of the upper conductor. The two conductors are then connected with one another by the solidified ejected material on the walls of a crater, which thus has the approximate form of a metallized hole. However, it will be appreciated that the uncontrollable ejection of melted material from the lower conductor located below the laser beam under identical conditions will result in different configurations of the linkage of the conductors in each crater. It follows that the links forms by this method under identical conditions exhibit different properties of electrical connection. Experience has confirmed the deficiencies of this method in terms of replicability and reliability.
Another known method for programmable laser connection of two superimposed conductors is described in particular in the article by J. I. Raffel et al., entitled "Laser Programmed Vias for Restructurable VLSI", published in the Technical Digest of the International Electron Devices Meeting, 1980, pages 132-135. This method comprises using amorphous silicon in the dielectric layer at the level of the connection point. The use of amorphous silicon in predetermined regions of the dielectric layer has the disadvantage that it complicates the manufacturing process of the integrated circuit and prevents the programming of any link by laser except in these regions.